To start your phonegap application begin with downloading phonegap for here .
A few things that must be considered when beginning your application:
First: How are you planning to compile your applications. If you are going to be creating an open source or are willing to pay for each compile then you can send your html file to phonegaps build servers and you will receive a nicely compiled program to distribute. However if you are planning on compiling the code yourself phonegap has provided us with nice documentation to get us started, here are the links .
To start my application I have decided to make use of a mix of jquerymobile and jquery, this allows for a lot of visual customization. For those of you that do not know what these are I suggest you go and look at the links i have provided.
So you have downloaded phonegap and jquery, now there is one more thing before we start, the thing is what platforms are we going to develop for? In my case I am developing for IOS,Android and Blackberry.